1. Field of the Invention
The present invention relates generally to the field of power controller apparatus for electrical and electronic equipment, and more particularly to time delay power controller apparatus for such equipment.
2. Background Discussion
It is well known by electrical engineers and most users of electrical or electronic equipment that when a piece of such equipment is turned on, a high turn-on electric current is usually caused in the equipment. Within several seconds, typically less than about 2-4 seconds, the turn-on current spike decays to a steady state operating current. The turn-on current spike is caused by the charging of cooperative elements and/or portions of the equipment and is dependent upon the rate of electrical charging of the equipment, as given by the equation: EQU I=dQ/dt,
wherein I is the current and dQ/dt is the time rate of charging (i.e., the charging rate).
From the above equation, it can be seen that a fast equipment turn-on time, for which dQ/dt is high, gives rise to a high current spike. Typically, the turn-on current spike is several times higher than the steady state or average current drawn by the equipment subsequent to turn-on.
The combined effect of individual high turn-on currents of each of several or a number of pieces of electrical or electronic equipment is that the current carrying capacity of an existing building electrical circuit into which the equipment is connected may be exceeded. This typically causes the associated electrical circuit breaker to trip open at the instant the equipment is turned on, thereby shutting down all the equipment on the circuit. This may occur even though the building circuit has the capacity for safely handling the combined steady state operating currents of all the equipment.
In some instances where several independent building electrical circuits are conveniently available, the possibility of overloading individual building circuits may be avoided by connecting different pieces of equipment into different circuits served by separate circuit breakers. However a multiplicity of separate building circuits is typically not available in a single room where a number of pieces of electrical or electronic equipment, for example, a computer and several associated disc drives, may be located. The installation of several independent building circuits to serve several different pieces of electrical equipment may be very costly and is typically impractical.
Although sometimes possible to do so, it is generally not feasible to substantially reduce turn-on current spikes by increasing equipment turn-on times (i.e., by increasing the "dt" term in the foregoing equation). Moreover, a slow rate of applying voltage may be damaging to many types of electrical equipment and motors and for safe operation requires the equipment to be specially designed.
As a consequence of high turn-on current problems associated with simultaneously turning on several pieces of electrical equipment, it is usually preferable to turn on just one piece of equipment at a time, with the interval between the turning on of successive pieces of equipment being sufficient to assure that the turn-on current drawn by a turned on piece of equipment has decayed to its normal operating level before the next piece of equipment is turned on. The following of such a time delayed turn on proceedure generally permits several pieces of electrical equipment to be operated from a single building circuit without overloading the circuit.
However, manual sequencing of several pieces of equipment is, itself, generally unsatisfactory because the required time interval between successive turn-ons is difficult to control. Also, it may be necessary or desirable to always follow the same, predetermined turn-on sequence for a particular system of interacting electrical equipment, and such sequencing may be difficult to assure by manual turn-on procedures. Out-of-sequence equipment turn-ons may cause system malfunctions, for example, loss of data in computer systems. It can, of course, be readily appreciated that specific turn-on schedules for particular electronic equipment systems may be desired or required even in situations in which high turn-on current spiking is not a problem or is not such a problem as to in and of itself require sequenced turn-on procedures.
Because of these and other turn-on problems, specialized power controller equipment has been developed. Exemplarly of advanced versions of such power controller apparatus is our above-identified prior application which discloses power controller apparatus having the capability for multiple time delayed turn-ons, and which thereby enables the operation of a plurality of different pieces of electrical or electronic equipment from a single power controller. Although the apparatus of our prior application has been determined to be very useful for controlling the turn-on of equipment in complex systems, such as computer or central processor systems, it does not provide for time delayed turning off of equipment as frequently may be desirable or necessary.
Even though sequential turning off of different pieces of interacting equipment is not required to reduce current spiking, it may be required to enable proper equipment shut-down schedules to prevent equipment damage, to prevent the loss of data or to prevent unsafe operating conditions. Thus, as an illustration, it may be necessary or desirable to shut down disc drives prior to turning off an associated central data processing unit in order to insure against the loss of computer data.
The present invention is, therefore, specifically directed to time delay power controller apparatus which provides time delayed power turn-off, with or without time delayed power turn-on being provided for the same equipment.